This invention relates to a control circuit for applying power from an electrical power source to a motor to control the motor over a wide range of speeds, and is more particularly concerned with such a power control circuit for applying a plurality of power pulses to a small fractional horsepower motor during each half cycle of the power source waveform with means of varying the duty cycle of the pulses over a wide range to correspondingly vary the speed and torque characteristics of the motor over a wide range without appreciable variation in the frequency of the power pulses.
It is difficult to control the speed of motors of less than one horsepower, and especially of motors less than 0.1 horsepower, particularly at slow speeds, due to the very low moment of inertia of the rotating components of these small motors. Since speed and torque of a motor are directly related variables, operation of the motor at slower speeds results in lower available torque. The control of slow speed operation of small fractional horsepower motors can become particularly acute when the low inertia is combined with other factors such as friction associated with motor brushes on the motor armature, or changes or disturbances in the load driven by the motor.
Various electronic circuits have been proposed to provide better control over multi-speed operation of motors. Thyristors, also known as silicon controlled rectifiers (SCRs), have been widely used for such purposes. Thyristors are most commonly utilized to pulse-width modulate the power source waveform to apply a selected portion of the power source waveform to control the motor speed or torque characteristics. However, since the torque output of motors controlled by thyristors, especially at less than half of rated speed, is directly proportional to the speed of the motor and to the current through the motor, thyristors are ineffective in controlling these motors at low speeds. This characteristic inability of thyristor control circuits to enable the motor to provide more uniform torque or to deliver more torque at lower speeds is independent of whether the thyristors are electrically connected to the motor in half-wave or full-wave bridge configurations.
Another disadvantage of thyristor controlled motors is that the power begins to be applied to the motor at a specific phase angle during the half cycle of the power source waveform and continues to be applied until the power source waveform changes from positive to negative or vice versa. At lower motor speeds, the power is typically applied to the motor for only a small portion of the half cycle of the power source waveform such that the motor has no power for 75 to 90 percent, or more, of the half cycle. For example, in a conventional 60 Hertz power source and with a half-wave thyristor control circuit, the motor may typically be without power for 10 to 15 milliseconds (ms.). These repetitive delays between energization of the motor, coupled with the low moment of inertia of fractional horsepower motors, cause such motors to rapidly decelerate or even stop between the applied power pulses. Despite the claims to a wide range of control over the motor speed, for example a range of 100 to 1, thyristor control circuits seldom exceed a practical range of speed control exceeding a range of 10 to 1 in small fractional horsepower motors.
Of course, thyristors are seldom used in D.C. circuits because of the difficulty and special techniques which must be used to stop current conduction in the thyristor. Transistors have been employed in such circumstances, and in A.C. circuits as well. Some of the transistor circuits attempt to regulate the amount of voltage applied to the motor and must therefore withstand high power dissipation at low speeds.
It is also known to the prior art to use transistors in motor control circuitry to apply pulses of power to the motor windings at frequencies above the usual power source frequency, as in variable speed stepping motors, to obtain incremental changes in motor position. These types of circuits employ either variable frequency pulse sources or vary the duty cycle of the pulses in a manner which causes the frequency of the pulses to change with change in the duty cycle. For example, one technique is to keep the on or off time of the pulse constant and to vary the duty cycle by increasing or decreasing the off or on time. This causes change in the period of the pulse, and hence the frequency.
In contrast, the present invention avoids appreciable power switching losses in the semiconductors which occurs at higher frequencies and applies a uniform number of power pulses to the motor during each half cycle of the power source waveform. It is therefore important to operate at a fixed frequency, such as a multiple of the power source frequency, and to vary the duty cycle of the constant frequency pulses, rather than varying the frequency of the pulses.